KisMAC is an open-source and free stumbler/scanner application for Mac OS X. It has an advantage over MacStumbler/iStumbler/NetStumbler in that it uses monitor mode and passive scanning.

KisMAC supports several third party PCMCIA cards - Orinoco, PrismII, Cisco Aironet, Atheros and PrismGT. USB Prism2 is supported as well, and USB Ralink support is in development. All of the internal AirPort hardware is supported as well.

Please note: This program is not intended for novices but for professional users. Take this as your warning, you should be prepared for advanced topics and know what you are doing with your system.